FIG. 1 illustrates a typical wind turbine 1 for use in large scale electricity generation on a wind farm. It includes a tower 2 and a wind turbine nacelle 3 positioned on top of the tower. The wind turbine rotor, comprising three wind turbine blades 5 connected to a hub 4, is connected to the nacelle through a main shaft (not shown in FIG. 1) which extends out of the nacelle front. Wind beyond a certain level activates the rotor due to lift induced on the blades and causes it to rotate. The rotation is converted to electric power, which is supplied to the electricity grid.
These tall wind turbines are located on exposed sites to maximize exposure to the wind, but they are also very exposed to lightning strikes which may cause extensive damage to a wind turbine. Thus, wind turbines are typically provided with a lightning protection system.
FIGS. 2a and 2b illustrate the typical lightning protection system in each of the wind turbine blades 5 (a single blade is shown in these Figures). The blades and, in particular, the tip of the blades are the most likely component of a wind turbine to be susceptible to a lightning strike as they are the component that project highest. Thus, the wind turbine blade 5 of FIG. 2a includes a tip with a metal receptor 8 that intercepts lightning strikes 7 that is grounded or earthed. The receptor is connected to a lightning down conductor 9 inside the wind turbine blade. The down conductor extends through the blade in the longitudinal direction and ends at the hub 4. FIG. 2b illustrates schematically one known arrangement in which lightning current is transferred from the lightning down conductor 9 to the rest of the lightning protection system and to ground. From the down conductor 9, the lightning current is transferred to the main shaft 10 of the wind turbine through a pitching mechanism 13 or any other mechanism between the blade and the shaft. In the nacelle 3, there are sliding contacts or slip rings in contact with the shaft in order to carry current from the shaft. The slip rings are connected to ground 11 by a down conductor extending through the wind turbine tower 1.
Problems with this type of arrangement are identified in International patent application No. WO 2005/050008 in the name of Vestas Wind Systems A/S (the same applicant/assignee as the present application). These include that the high energy of the lightning current passing through different components of the wind turbine such as the blade pitching mechanism 13 may damage these components and that the slip ring arrangement is inefficient.
International patent application No. WO 2005/050008 describes an improved lightning current transfer unit to address these problems. It is illustrated in FIGS. 3a to 6.
The lightning current transfer unit 15 of FIG. 3a forms an electrical connection between a lightning down conductor of each blade 5 of the rotor via an electrically conducting ring or blade band 18 around the outside of each blade of the wind turbine and a lightning down conductor of the nacelle 3 via an electrically conducting ring or lightning ring 16 on the nacelle.
As shown best in FIG. 3b, the lightning current transfer unit 15 is mounted on a protruding portion 21 of the hub 4 on the inside of the hub facing the nacelle 3. The lightning current transfer unit 15 projects outwardly between the inside surfaces of the hub and nacelle in a space between the wind turbine blade and the front of the nacelle. As the lightning connection means or lightning current transfer unit is mounted to the hub, it rotates with the hub.
Referring back to FIG. 3a, the electrically conducting or metal ring 18 around the outside of each blade 5 surrounds the root of the blade. Each ring forms a contact surface 18 on the root of the wind turbine above the pitching mechanism and perpendicular to the longitudinal direction of the blade. The contact surface thus rotates with pitching of the blade. Each ring is connected to the lightning down conductor 9 inside the wind turbine blade as described above.
The electrically conducting or metal ring 16 on the outside of the nacelle 3 facing the hub 4 is mechanically connected to the nacelle. It is electrically connected to a lightning down conductor 14 of the nacelle. The metal ring 16 forms a contact surface 17 to the lightning current transfer unit 15.
FIGS. 3b and 3c closer illustrate the position of the lightning current transfer unit 15 in relation to the contact surface 18 on the wind turbine blade and the contact surface 17 on the nacelle. It also illustrates the different sections of the lightning current transfer unit 15, which include a base support part 22, two contact means 19a, 19b and flexible links 26, 27 between the base support part and the two contact means. The flexible links ensure that the two contact means are forced against the contact surface 18 on the wind turbine blade 5 and the contact surface 17 of the nacelle 3, respectively. The two contact means 19 and the two contact surfaces 17,18 establish two contact areas 20a, 20b. 
The first of the contact areas 20a ensures a constant contact to the electrically conducting ring 18 of the blade 5 and the other 20b to the electrically conducting ring 16 of the nacelle 3. The two contact means 19a, 19b are connected by a dedicated electric connection 30 in the form of a wire or cable.
Each of the contact means 19a, 19b also comprises a retaining bolt 28, 29 for the dedicated electric connection 30 allowing the electric connection to be established and retained between the two contact means. The electric connection 30 is made of a flexible material with a length corresponding to the distance between the two contact means at their rest position, when they are furthest apart. If the lightning connection means or lightning current transfer unit 15 is exposed to forces the flexible link will bend resulting in a more sagging dedicated electric connection 30.
FIG. 4 illustrates schematically the contact areas of the lightning connection means or lightning current transfer unit sliding on the conducting ring or lightning ring 17 of the nacelle 3. It illustrates the situation of a rotating three-bladed wind turbine rotor with three lightning connection means including contact means 20b. As each lightning connection means is mounted on the hub and main shaft 14 they will rotate with the main shaft as the centre of rotation. Further, the contact means is positioned at a distance from the centre corresponding to the diameter of the ring. The contact means will thus perform a circular rotation facing the ring while being continuously forced against the surface of the ring.
FIG. 5 illustrates the contact surface 18 of the blade 5. The contact means 20a is continuously forced against the contact surface or blade band and slides on the surface when the blade is pitched to one or the other side.
While this arrangement is effective as a lightning current transfer unit, it does not offer adequate electromagnetic compatibility (EMC) with other components of the wind turbine.